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Stupid question - focal length and hyperfocal distance
Old 05-21-2010   #1
Ezzie
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Stupid question - focal length and hyperfocal distance

I suspect I know what the answee will be, but I need to ask the question none the less. What is the correct value for calculating hyperfocal distance?The reason I'm asking is that my Fujinon 90/f8 has a flange to focal plane distance of 99.2mm. Which is quite a bit off the given value of 90mm. At f16 the difference for the hyperfocal distance is over 1.5m for a 6x12 format camera! My DIY LF P+S has a helicoid, so I can adapt, but if I'd built it fixed focus I could have been in for a surprise?
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Old 05-21-2010   #2
antiquark
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It sounds like you're trying to determine the "true" focal length of the lens. This page might be helpful to you:
http://www.bobatkins.com/photography...al_length.html

(Also check out dofmaster.com)
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Old 05-21-2010   #3
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I think more than one of us would like to see your DIY LF P+S camera. Give us a look.
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Old 05-21-2010   #4
Rob-F
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I had to read the post over six times, trying to understand the reasoning. I don't think the flange to film plane distance is relevant for your problem; or perhaps has little to do with hyperfocal distance.

There is no reason why the flange to focal plane distance can be expected to be the same as the lens focal length. The focal length is the distance from the nodal point, or principle plane, to the focal plane. The nodal point can be almost anywhere the designer places it. In a retrofocus lens it can be behind the lens, completely outside the physical space of the lens. In a telephoto lens, it can be out in front of the lens. Or, when there is no requirement for it to be otherwise, it can be somewhere within the lens barrel. In your case, it sounds like the nodal point is outside and behind the lens barrel. None of this is much help for determining hyperfocal distance.

In his book, Optics, Arthur Cox gives Hyperfocal distance as:

H=f + 1000f/N

where H is the hyperfocal distance in inches, f is focal length, and N is the f-number.

It occurs to me that maybe what you wanted to was to determine the lens extension at some hyperfocal distance you have already decided upon. I wonder if that might be best done by experimentation, just because of the difficulty of not knowing where to physically measure to on the lens. But if you need that, I know I can dig it out of Cox's book, since I have been through this before over extension tubes.

Edit: It is clear that Cox is assuming some given size for the circle of confusion, in his formula. He mentions .004 inch in the text. That may or may not do for your purposes.
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Last edited by Rob-F : 05-21-2010 at 17:32.
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Old 05-21-2010   #5
payasam
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Focal length matters, not film-to-flange distance, and helicoids are good for immortal and all other souls. You should have the flexibility to adapt your home brew device to different apertures, which will of course change the near limit of the hyperfocal distance.
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Old 05-21-2010   #6
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Thanks Rob and Derek. I forgot the obvious. The process of collimating the lens to film plane has given me tunnel vision, that and the fact that I've been interested in establishing a feel for zone and hyperfocal focusing, as will be applied to the aforementioned DIY effort. The helicoid has distance scales, but no DOF scales. (If all else fails I can still use it as a view camera. )

I'll read up on your links, good thing I've still a lot to learn.

Regarding the camera itself. There's a thread on the Large Format RF sub-forum on the issue. Describes my process from a vague idea I wanted a 617 panoramic camera, and why I ended up building a 4x5/6x12 instead. Pictures of the camera start towards the end of page 2 (versions 0.1 and 0.2, and parts overview) and page 3 (current version 0.3)

http://www.rangefinderforum.com/foru...5548&page=1%27

Current incarnation:
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Old 05-21-2010   #7
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Quote:
Originally Posted by payasam View Post
Focal length matters, not film-to-flange distance, and helicoids are good for immortal and all other souls. You should have the flexibility to adapt your home brew device to different apertures, which will of course change the near limit of the hyperfocal distance.
Yes, thanks. I've been playing around with a DoF calculator, f16 on a 90mm with a film size (nominal) of 6x12 which yields a near limit of 3.3m and far limit of infinity. Hyperfocal distance 5.90m. F22 gives 2,72m, infinity and 4.32m respectively.
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Old 05-22-2010   #8
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Ezzie, I have two depth of field calculators, both spreadsheets, which I'll gladly send you if you give me an e-mail address. Your rig looks well made, though not much about its construction is readily apparent. May I ask what kind of work you do with it? The "P&S" might be the answer, but then one would have to wonder what's wrong with 35mm and digital.
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Old 05-22-2010   #9
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That is an impressive rig, Ezzie.

You can use tables or dofmaster to calculate DOF.

To measure the exact focal length, I would measure the FOV, and then go backwards.

Roland.
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Old 05-22-2010   #10
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Thanks for showing and thanks for the 2/10 thread, somehow I missed that. Those are some great looking cameras.
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Old 05-22-2010   #11
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Quote:
Originally Posted by payasam View Post
Ezzie, I have two depth of field calculators, both spreadsheets, which I'll gladly send you if you give me an e-mail address. Your rig looks well made, though not much about its construction is readily apparent. May I ask what kind of work you do with it? The "P&S" might be the answer, but then one would have to wonder what's wrong with 35mm and digital.
Thanks for your input. I´ve a calculator for the iPhone already, but if I find it inaccurate I´ll let you know.

The idea started with a notion of panoramic photography, but not dull digital stitching. As most cameras for this purpose are extortionately expensive, either they are super 35mm (xpans) or MF. I thought: What the heck, I´ll make my own. OK, I´ll brew my own as some would say - since I´ll not be making the lens, film holder and shutter (DIY die-hards make all these and despise those that deem to call their rigs DIY if not all of the above boxes are ticked). I contemplated this and that, but ended up wanting at least some flexibility with regards to use. I wanted MF from the start, or even LF. Using LF lenses makes thing easy, with leaf shutters and all. Interchangeable film holders etc. 6x17 as I initially intended building just seemed big and expensive. 4x5 LF and all the kit available for it (at a reasonable price) seemed more sensible (how sensible is building your own camera?). I found it hard to incorporate movements, and ended up with what can be called a MF/LF P+S. It´ll no doubt be used on a tripod most of the time, but then again I´m looking forward to more slow and contemplative photography.

Did I mention I also wanted to get back into film? Well that was part of the parcel too.

A picture describing the construction somewhat:
http://www.rangefinderforum.com/phot...ERR0650-wt.jpg

Or you can follow the thread I linked to earlier.

P.S The whole lens box is detachable, making it possible to mount wider lenses on the same frame and back. Also will have digital sliding back for DSLR on the rig, which will require a lens box somewhat 55-60mm shallower than the one used here.

Now I regret not making a field camera instead, might have been easier
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Old 05-22-2010   #12
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Quote:
Originally Posted by charjohncarter View Post
Thanks for showing and thanks for the 2/10 thread, somehow I missed that. Those are some great looking cameras.
I agree. That´s part of the attraction. We´ll see how the camera (and more importantly how I) perform sooner than later I hope.

It´ll probably not be my last DIY effort, we´ll see.
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Old 05-22-2010   #13
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Quote:
Originally Posted by ferider View Post
That is an impressive rig, Ezzie.

You can use tables or dofmaster to calculate DOF.

To measure the exact focal length, I would measure the FOV, and then go backwards.

Roland.
Thanks Roland. Both for the tip and the praise.
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